MU-MIMO (Multiple User-Multiple Input Multiple Output)) is an important means to increase peak data rate and spectral efficiency in current wireless communications. In general, it is possible to support MU-MIMO operation with SU-MIMO (Single User-MIMO).
FIG. 1 shows a scenario that a transmission point updates CQI (Channel Quality Indicator) and PMI (Precoding Matrix Index) of SU-MIMO for MU-MIMO operation.
As shown in FIG. 1, two user equipments (UEs) may report two sets of SU PMI/CQI, respectively. For example, UE1 reports SU PMI1/CQI1 to the transmission point, and UE2 reports SU PMI2/CQI2 to the transmission point. After receiving the SU PMIs and CQIs, the transmission point will update the SU PMI1/CQI1 and SU PMI2/CQI2 internally to have the PMIs and CQIs feasible for MU transmission, and send data by PDSCH (Physical Downlink Shared Channel) using MU PMI1/CQI1 and MU PMI2/CQI2 to the UE1 and UE2 respectively, as shown in FIG. 1.
However, such update at the transmission point is not accurate. For example, the CQI respectively associated with each UE may be degraded for the MU transmission, compared with SU transmission. The degradation is different for different user equipment implementations. For example, some user equipments may have advanced interference rejection capability, and their CQI degradation can be marginal. For some other user equipments that have less advanced features, the CQI degradation can be significant. However, the internal update at the transmission point can not reflect the difference, because in general the transmission point is not aware of user equipment implementations. Therefore, the internal update at the transmission point is not accurate.
FIG. 2 shows a scenario that intended user equipment reports BCI (Best Companion Precoding Matrix Index) and MU-CQI to the transmission point.
To enhance CQI accuracy for the MU transmission, currently BCI and MU-CQI are being discussed in the LTE-A standardization. To report the BCI to the transmission point, the intended user equipment firstly assumes that the potentially paired user equipment adopts a certain PMI (i.e., BCI) causing small interference to the intended user equipment. Then, the intended user equipment calculates the CQI according to the assumed BCI, which is MU-CQI. Finally, the assumed BCI and the calculated MU-CQI are reported to the transmission point in addition to SU PMI and CQI.
Although the assumed BCI and the calculated MU-CQI help MU operation at the transmission point, but it still needs to be updated at the transmission point, otherwise the throughput performance of the paired user equipment will be severely degraded.
FIGS. 3(a) and (b) shows a scenario that the actual transmission and the assumed transmission are not matched. As shown in FIGS. 3(a) and 3(b), the assumed BCI1 at the intended user equipment (UE1) for possibly paired user equipment (UE2) as shown in FIG. 3(a) does not match the actual PMI2 used for the paired user equipment (UE2) as shown in FIG. 3(b).